Luminescence is a phenomenon in which the electronic state of a substance is excited by an external energy source and emits this energy in the form of light when it returns to its grounded state. Photoluminescence is the one form of the luminescence in which the excitation energy source is incident light and it includes both fluorescence and phosphorescence. These two phenomena are fundamentally different and are substantially different with respect to their lifetime. For inorganic materials, light emission from a substance during the time when it is exposed to exciting radiation is called fluorescence, while after-glow if detectable by the human eye after the cessation of excitation is called phosphorescence. For organic molecules, light emission from a single excited state is called fluorescence, while that from a triplet excited state is defined as phosphorescence.
Phosphor, which is a solid luminescent material, has a wide range of applications classified as: (1) light sources represented by fluorescent lamps; (2) display devices represented by cathode-ray tubes; (3) detector systems represented by x-ray screens and scintilators; and (4) other simple applications, such as luminous paint with long persistent phosphorescence.
Most phosphors are composed of a transparent microcrystalline host (or a matrix) and an activator, i.e., a small amount of intentionally added impurity atoms distributed in the host crystal. Different combinations of host and activators give rise to different characteristics such as color, the degree of initial luminescence intensity, and luminescence decay properties.
Sulfide phosphorescent phosphors including CaS:Bi (violet blue), CaStS:Bi (blue), ZnS:Cu (green), and ZnCdS:Cu(yellow or orange) have been known nearly 100 years. However, (Ca, Sr) S:Bi phosphor (blue) shows extremely poor chemical stability of the host material as well as weak luminance and after glow characteristics. CaSrS:Br3+ is produced by adding Bi3+ to a mixture of CaCO3, SrCO3, and S and then heating to 1100° C. in normal atmosphere for 1.5 hours. However, it is rarely used as a phosphorescent medium since it decomposes readily when exposed to moisture. A red-emitting phosphor, ZnCdS:Cu is not practically used since Cd, which occupies almost a half of the host material is highly toxic. A green-emitting phosphor ZnS:Cu is the most widely used phosphor and is inexpensive. It is produced by adding Cu, 10−2wt % of the weight as the activator to ZnS, mixing with flux (NaCl, KCL, or NH4Cl, etc.), and then heating to 1250° C. for 2 hours in a normal atmosphere. In addition to Cu, several parts per million (ppm) of Co may also be added. However, zinc sulfide phosphorescent phosphor is decomposed as the result of irradiation by ultraviolet radiation in the presence of moisture and thus blackens or reduces the luminance. Therefore, it is difficult to use this phosphorescent phosphor in fields where it is placed outdoors and exposed to a direct sunlight, thus limiting its application to luminous clocks/watches and instrument dials, excavation guiding signs or indoor night time displays. Normally, after-glow time is between about 30 minutes to 2 hours (see U.S. Pat. Nos. 5,424,006 and 5,951,915).
The relatively new categories of phosphor, alkaline earth metal type aluminate phosphor, overcome many shortcomings of the sulfide phosphors. One such example is the new phosphor SrAl2O4:Eu2+, Dy3+ invented by Nemoto & Co. Ltd in 1993 see U.S. Pat. No. 5,424,006. This material is produced by mixing Al2O3 and SrCO3, adding Eu2+ and Dy3+ as the activator and co-activator, respectively, and then heating it in a reducing atmosphere electric oven to 1300° C. for 3 hours. SrAl2O4:Eu2+ emits a broadband green luminescence peaking at about 520 nm due to the 4f-5d transition of Eu2+, and has long after-glow persistence. This alkaline earth metal-type aluminate activated by europium or the like is a novel phosphorescent phosphor completely different from conventional sulfide phosphorescent phosphors. Further, it was shown to be chemically stable and showed excellent photo-resistance due to an oxide. Adding Dy3+ as the auxiliary activator dramatically increases the initial brightness.
The more general form of alkaline earth metal-type of aluminate phosphors is:MA12O4:Eu,(N)
wherein:
M=at least one metal element selected from calcium, strontium, barium
Eu: 0.001%–10% (an activator)
N: as a coactivator, 0.001–10%, at least one element selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, tin and bismuth.
Other types of Eu-activated phosphor have also been developed and show different luminescence color and properties. One example is a Eu-activated silicated phosphorescent phosphor (see U.S. Pat. No. 5,951,915).
The presently known long phosphorescent phosphors are listed in Table 1 below. In this table, the luminance values of the phosphors are reported for samples with a thickness of more than 200 mg/cm2, measured 10 minutes after a 5-minute exposure to a 1000−1×(D65) light source (according to Japanese Industrial Standard, JIS Z 8720, Standard Illuminants and Source for Colorimetry), whose color temperature is 6500K. Persistent time refers to the time (in minutes) that it takes for the after-glow to decrease to a luminance of 0.3 mcd/m2 representing the lower limit of light perception of the human eye.
TABLE 1(Luminous Phosphors)After-glowbrightnessLuminescence(afterAfter-glowLuminescencewavelength at10 min)persistence timeCompositioncolorpeak (nm)(mcd/m2)(min)CaSrS:Br3+(Sr, 10–20%)Blue4505Semi-long (about 90)CaAl2O4:Eu2+, Nd3+Blue44035Long (over 1000)ZnS:CuYellow-Green53045Semi-long (about200)ZnS:Cu, CoYellow-Green53040Long (over 500)SrAl2O3:Eu2+Green52030Long (over 2000)SrAl2O3:Eu2+, Dy3+Green520400Long (over 2000)CaS:Eu2+, Tm3+Red6501.2Short (about 45)
Incorporating photoluminescent phosphor into textile structures provides major advantages in many uses, especially in safety applications. In the past, this photoluminescence effect has been especially useful for the marking of emergency pathways. Escape routes that are marked with photoluminescent products on the floor and at the lower part of the wall remain visible for many hours even in power failure situations. The desire to use this photoluminescent effect for protective clothing led to increasing interest in photoluminescence textile goods development. Athletic apparel, hunting gear, ropes and cords, life vests and even carpets for theaters and airplane interiors are a few examples. Other applications may include lingerie, and protective clothing markets for firefighters and chemical workers. However, incorporating phosphorescent pigment into textile structures to provide enough durability, luminescence intensity, and good after-glow properties without impairing the physical properties has been a unique challenge in producing photoluminescent textile goods.
Photoluminescent phosphors also have been applied to yarns by passing them through a bath containing a photoluminescent material and a binder (see U.S. Pat. Nos. 2,787,558 and 3,291,668). Such methods, however, may lead to increased stiffness of the yarn and fabrics, loss of textile-like properties and vulnerable to abrasion. Consequently, the properties of the textiles formed from such yarns are inadequate and the durability of their photoluminescence is normally poor.
To improve the photoluminescence of textile properties in yarns, direct spinning of photoluminescent homocomponent fibers has also been attempted. Photoluminescent polymers can be made by mixing and kneading of a thermoplastic polymer and photoluminescence phosphors (see U.S. Pat. No. 6,123,871) and this polymer can be subsequently extruded into fibers (see U.S. Pat. Nos. 5,674,437 and 5,914,076). Although, direct incorporation of the photoluminescence phosphors into fibers overcomes many of the difficulties with coating methods, many challenges remain. When a luminous fiber is prepared by a method which comprises kneading aluminous pigment directly into a fiber, the content of the luminous pigment is preferably 5% by weight or less. When the content exceeds 5% by weight, fiber-forming characteristics of the polymers tend to deteriorate. Consequently, the fibers will be more brittle, cannot be drawn easily to the same extent as the pure polymer and are significantly weaker than their pure polymer fibers. Further, over time, the moisture that can be present on the surface and the circumference of the fiber may react with the luminous pigment and cause discoloration and deterioration of the luminous performance. It has been revealed that such phenomena will shift gradually from the surface to the inside of the fiber with the luminous pigment exposed on the fiber surface acting as a trigger.
In prior art, bicomponent sheath/core fiber was used to enhance fiber-forming properties. A high luminance luminous fiber comprising a core component containing a polyolefin resin and a luminous pigment and a sheath component comprising a polyolefin resin containing no luminous pigment is the subject of U.S. Pat. No. 6,162,539. The luminescent material content and core/sheath ratio was shown to be critical for both luminescent properties and fiber forming properties. The patent discloses that the core component may contain up to 60% by weight of the luminous pigment. It has been reported, however, that when the core to sheath ratio was less than 1:3, section unevenness tended to develop in the core and that this tended to deteriorate fiber-forming properties. Similarly, when the core to sheath ratio exceeded 1:1, the fiber strength tended to decrease significantly.
The present invention is intended to overcome many of the well known deficiencies of prior art luminescent fibers and to provide a new and improved photoluminescent fiber.